1. Field of the Invention
The present invention relates to a passive optical network, and in particular to the recovery of information from signals transmitted from terminations to a head-end station in such a network.
2. Related Art
The present applicant has developed a bit transport system (BTS) for use in a TPON (telephony on a passive optical network) network. In this BTS a head-end station broadcasts time division multiple access (TDMA) frames to all the terminations on the network. The transmitted frames include both traffic data and control data. Each termination recognises and responds to an appropriately addressed portion of the data in the broadcast frame and ignores the remainder of the frame.
In the upstream direction, each termination transmits data in a predetermined time slot and the data from the different terminations are assembled at the head-end into a TDMA frame of predetermined format.
One feature necessary to such a network is the provision of compensation for the differing delays and attenuation associated with the different distances of the various terminations from the head-end station. To this end, in the BTS each termination is arranged to transmit a ranging pulse timed to arrive in a respective predetermined portion of the upstream TDMA frame. The head-end station is arranged to monitor the timing, i.e. phase, and amplitude of the arrival of the pulse from each of the terminations and to return a servo-control signal to the termination to retard or advance its transmissions as appropriate and to adjust the launch power. This active fine ranging enables the BTS to ensure the stability of the upstream TDMA frame and, for example, to compensate for fluctuations in timing and received power due to such effects as changes in the operating temperature of the network. However this places severe demands on the design of the head-end station, requiring the measurement of the timing of received signals to within a fraction of a clock cycle and amplitude to within a fraction of a dB in real time.